The present invention relates generally to automatic frequency control apparatus, and, more particularly, to an automatic frequency control system for correcting frequency differences between a receiver and a transmitter which transmits either an analog information signal or a discrete, encoded information signal.
An information signal is impressed upon an electromagnetic wave by a process referred to as modulation. In a modulation process, the information signal is combined with an electromagnetic wave (referred to as the carrier wave), and the resultant, combined signal is an electromagnetic wave which varies in some manner according to the values of the information signal. Various modulation techniques have been developed to modulate the information signal upon the electromagnetic wave; amplitude modulation, frequency modulation, and phase modulation are three of such modulation techniques.
In general, an amplitude modulation signal is formed by modulating the information signal upon the electromagnetic wave such that the information signal modifies the amplitude of the electromagnetic wave corresponding to the value of the information signal. The frequency of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the shape, or amplitude, of the signal. The shape of the signal is referred to as the envelope of the signal, and the changes in the amplitude of the modulated signal change the envelope formed thereby. A frequency modulated signal is formed by altering the frequency of the electromagnetic wave corresponding to the value of the information signal. The amplitude of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the variation of the frequency of the signal. A phase modulated signal is formed by altering the phase of the electromagnetic wave corresponding to the value of the information signal. The amplitude of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the variation of the phase of the signal. Because the amplitudes of a frequency modulated and a phase modulated signal do not vary, these modulated signals are referred to as constant envelope signals.
A receiver which receives the modulated information signal includes circuitry to detect, or to otherwise recreate, the information signal modulated upon the electromagnetic wave. This process is referred to as demodulation, and various receiver circuits permits demodulation of information signals modulated upon an electromagnetic wave according to the various modulation techniques.
Many different modulated information signals may be simultaneously transmitted by a plurality of transmitters at a plurality of different frequencies.
Portions of a 100 megahertz band of the electromagnetic frequency spectrum (extending between 800 megahertz and 900 megahertz) are allocated for radiotelephone communication, such as, for example, by radiotelephones utilized in a cellular, communication system. A radiotelephone contains circuitry both to generate and to receive modulated information signals.
A cellular, communication system is created by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each of the base stations is constructed to receive and transmit modulated information signals simultaneously to and from radiotelephones to permit two-way communication therebetween. The base stations are positioned at locations such that a radiotelephone at any location in the geographical area is within the reception range of at least one of the base station receivers.
The geographical area is divided into portions, and one base station is positioned in each portion. Each portion of the geographical area defined thereby is referred to as a "cell".
As mentioned hereinabove, a portion of the 100 megahertz frequency band is allocated for cellular communications. Although numerous modulated information signals may be simultaneously transmitted at different transmission frequencies, each occupies a finite portion of the frequency band. Overlapping of simultaneously transmitted modulated, information signals is not permitted as interference between overlapping signals on the same frequency could prevent detection of either of the modulated information signals by a receiver.
The frequency band is divided into channels, each of which is of a thirty kilohertz bandwidth. Presently, one signal is permitted to be transmitted in each thirty kilohertz-wide channel of the frequency band. Additionally, a first portion, extending between 824 megahertz and 849 megahertz, of the frequency band is allocated for the transmission of modulated information signals from a radiotelephone to a base station. A second portion, extending between 869 megahertz and 894 megahertz of the frequency band is allocated for the transmission of modulated information signals from a base station to a radiotelephone. 832 transmission channels are formed in the first frequency band portion, and 832 transmission channels are formed in the second frequency band portion, thereby permitting a maximum of 832 simultaneous, two-way communications within a geographical area.
A modulated signal transmitted upon any one of the transmission channels must be of a bandwidth less than the bandwidth of the transmission channel (i.e., less than thirty kilohertz). Oscillators which oscillate at frequencies to generate the electromagnetic carriers thereby are susceptible to frequency variations. Such variations, referred to as frequency drift, can cause the transmitted signal to extend beyond the boundaries of the transmission channel.
Increased usage of the cellular, communication systems has resulted, in many instances, in full utilization of every transmission channel allocated for cellular, radiotelephone communication. Other frequency bands of the electromagnetic spectrum are similarly oftentimes fully utilized.
Various attempts have been made to increase the information-transmission capacity of the cellular, radiotelephone communication systems as well as other communication systems utilizing other frequency bands of the electromagnetic spectrum. However, existing cellular radiotelephone communication systems are comprised of radiotelephones and base stations having circuitry which transmits and receives frequency modulated, analog signals. Only one modulated information signal may be transmitted upon a transmission channel at a time. Significant increases in the information-transmission capacity of cellular, radiotelephone communication systems has accordingly, been limited.
Discrete modulation techniques have been developed, however, to permit transmission of more than one signal at the same frequency. A cellular, radiotelephone communication system capable of transmitting modulated information signals formed by discrete modulation techniques would allow transmission of more than one signal on a transmission channel. The capacity of such a communication system can be significantly increased.
In general, a discrete modulation technique encodes a continuous, information signal into discrete signals and then modulates the discrete signals upon an electromagnetic wave to form thereby a modulated information signal. Discrete signals of more than one information signal may be modulated upon electromagnetic waves of identical carrier frequencies and transmitted sequentially to two or more radiotelephones.
Frequency drift may be a greater problem when certain discrete, encoded signals are transmitted. Oscillators which generate electromagnetic waves upon which information signals are modulated are susceptible to frequency drift responsive to changes in ambient conditions, such as, for example, temperature changes and supply voltage variations. A frequency drift of a magnitude which maintains a conventional, analog signal within the boundaries of a transmission channel, may oftentimes be of a magnitude which causes a corresponding discrete, encoded signal to extend beyond the boundaries of a transmission channel. Frequency drift of modulated information signals generated by discrete modulation techniques may be more susceptible to interference problems than are modulated information signals generated by conventional analog modulation techniques.
Additionally, circuitry for demodulating transmitted information signals of certain discrete, encoded-type signals requires less frequency error than the frequency error permitted of conventional, analog signals. Quantitatively, the U.S. cellular standard frequency error permitted of conventional, analog signals is 2.5 parts per million, whereas frequency error permitted of discrete, encoded signals is approximately 0.2 parts per million.
Systems and methods of frequency control for minimizing frequency drift to minimize thereby frequency drift problems are known and are frequently utilized in many existing communication systems. Generally, one oscillator, referred to as the reference oscillator, within a transmitter is controlled such that frequency drift of a signal generated thereby is within an allowable range. Other oscillators of the radio may then be locked to the frequency of the reference oscillator.
In the particular instance of the cellular, radiotelephone communications as above-described, the oscillators of the base stations positioned throughout the geographical area to modulate an information signal thereupon may be precisely controlled to minimize drift of the frequency of the electromagnetic wave generated thereby. The receivers of the radiotelephones may utilize the frequency of the modulated information signal from the base station as a reference frequency. The reference frequency is utilized by the radiotelephone, for example, as reference from which the transmit frequency of the radiotelephone may be offset allowing the signal transmitted by the radiotelephone to be as precise of a frequency as the base station frequency.
In order to increase the capacity of cellular, communication systems, existing base stations having circuitry to transmit and receive only conventional analog signals are to be converted to base stations which additionally permit transmission and reception of discrete, encoded, modulated information signals. Radiotelephones are being developed to permit transmission and reception of both conventional analog signals, and discrete encoded signals. As the cellular system base stations are gradually converted, and radiotelephones are similarly developed, some channels of a cell will be comprised of receivers having circuitry permitting reception of discrete, encoded modulated information signals, and other channels will be comprised of receivers having circuitry permitting only reception of conventional analog modulated information signals. Similarly, some radiotelephones operated in the cellular, communication systems will contain circuitry permitting transmission and reception of both discrete, encoded modulated information signals and conventional, analog modulated information signals. Other radiotelephones will contain circuitry permitting transmission and reception of only conventional, analog, modulated information signals.
A dual-mode radiotelephone permitting transmission of both conventional, analog modulated information signals, and discrete, encoded modulated information signals may be constructed having both first circuitry for transmission and reception of the conventional, analog modulated information signals, and second circuitry for transmission and reception of discrete, encoded modulated information signals. When a radiotelephone receives a discrete encoded signal, a digital signal processor may be conveniently utilized to decode the signal. At the same time, the digital signal processor may be utilized to derive an error signal to correct the reference frequency in the radiotelephone.
While a digital signal processor may be utilized to generate the error signal to correct the reference frequency when an analog signal is received, the digital signal processor dissipates larger amounts of power than conventional analog circuitry used to determine the reference frequency of a conventional, analog modulated information signal. The conventional, analog circuitry is, however, unsatisfactory for determining a reference frequency for discrete encoded, information signals.
A radiotelephone operable to receive both conventional analog and discrete, encoded, modulated information signals having circuitry to determine a reference frequency of either type of transmitted signal, and, additionally, having minimal power consumption requirements would be advantageous.
What is needed, therefore, is a frequency control scheme which requires minimal power consumption, but also may be alternately operated to determine the reference frequency of either conventional, analog modulated information signals or discrete, encoded modulated information signals transmitted to the radiotelephone.